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Abstract

Despite the importance and broad understanding of fitness as a concept in evolutionary biology, there remains wide disagreement about the ways in which fitness is described and measured in the field. In particular, fitness is generally taken to refer to the contribution of genes to future generations relative to the contribution of others. However, field biologists generally measure individuals, not genes. Moreover, there is a problem of scale and what exactly is meant by "future generations." Should the appropriate measure of fitness be the number of offspring, the number of reproductive offspring, or the reproductive success of those offspring? It is generally assumed that measuring the number of offspring and the number of grand-offspring will give essentially equivalent answers, but this is not necessarily the case. These questions are complicated by the time-limited nature of most field studies. Only truly long-term data sets can be used to measure the effects of covariates on the production of offspring and grand-offspring, but this type of study is difficult to complete. It would, therefore, be useful to know to what extent long term patterns can be predicted from a single year's data. This study examines the determinants of annual and lifetime reproductive success in female yellow-bellied marmots (Marmota flaviventris) measured at three different time points: the number of offspring produced, the number of reproductively competent two-year-old daughters produced, and the number of grand-offspring produced. Using a 40-year data set, I examine the social and environmental variables present during the year of the mother's reproduction to predict these measures of reproductive success. Thus, although I am using a very long-term data set, I am testing the feasibility of making accurate predictions using only one year's data.